Ethylene/butene-1 copolymers

This application is a continuation of application Ser. No. 378,484, filed July 12, 1989, which is a continuation of application Ser. No. 028,013, filed Mar. 18, 1987, which in turn is a continuation of application Ser. No. 874,781, filed June 12, 1986, which in turn is a continuation of application Ser. No. 795,747, filed Nov. 7, 1985, which in turn is a continuation of application Ser. No. 665,085, filed Oct. 26, 1984, which in turn is a continuation of application Ser. No. 198,592, filed Oct. 20, 1980, which in turn is a continuation of application Ser. No. 062,625, filed Aug. 1, 1979, all now abandoned.

Low and mean density polyethylenes (values generally lower than 0.945) obtained by low pressure polymerization processes in which ethylene is polymerized in admixture with an alpha-olefin, in particular propylene or butene, in the presence of Ziegler-type catalysts or of catalysts based on silica-supported oxides, are already known.

The amount of polymerized alpha-olefin contained in those polymers generally ranges from 1 to 15% by moles. The ratio between the molar percentage of alpha-olefin and the polymer density generally is not lower than 5-6 for density values equal to about 0.910.

A high content of alpha-olefin in the polyethylene chain on one side contributes to a lowering of the polymer density and, on the other side, adversely effects all the mechanical properties of said polymer.

Thus, there has been a need for polymers which, for a given density, have a minimum content of alpha-olefins.

Copolymers endowed with said characteristics (low density and simultaneously low content of alpha-olefin) are described in Canadian Patent No. 849,081. Those copolymers, however, are characterized by a relatively low melting point, comprised between about 122° and 102° C. for density values decreasing from about 0.930 to 0.910 g/cc. In the field of the copolymers which, for a given density, have a low content of alpha-olefin, it is desirable for insuring good mechanical properties, that the polymer melting point be as high as possible.

One object of this invention is to provide new low-density copolymers of ethylene and the alpha-olefins containing a small amount of the polymerized alpha-olefin, having a high melting point and excellent mechanical properties.

That and other objects are achieved by the present invention in accordance with which it has been found, surprisingly, that it is possible to prepare crystalline copolymers of ethylene with alpha-olefins CH 2 ═CHR, wherein R is an alkyl radical with 2 to 8 C, having a density below 0.945 g/cc, in particular ranging from 0.915 to 0.930 g/cc, containing 0.5 to 3% by moles of polymerized alpha-olefin. In these copolymers, the ratio between molar percentage of alpha-olefin and polymer density reaches, at the most, a value of 3.5 when the density values are approximately equal to 0.910 and decreases to values of about 0.5 for density values around 0.945. The melting point of the copolymers is comprised between 110° and 130° C., and increases as the density increases.

For a given density and a given content of alpha-olefin, the copolymers of the present invention are therefore characterized, as compared to the polymers known so far, by a higher melting point. That results in better mechanical properties and in particular better tenacity.

Another characterizing feature which differentiates the present copolymers from those known heretofore consists in that the number of methyl groups per 100 Cs is almost equal to the number of radicals deriving from the alpha-olefin.

For a given content of alpha-olefin the density of the copolymers varies according to the melt index of the copolymer (determined according to standard ASTM 1238 cond. E).

However, the corrections in respect of the density of the copolymer having a melt index equal to 1 are relatively small, as they refer only to the third decimal figure. The melt index is respectively decreased or increased depending on if it is higher or lower than 1.

For a copolymer containing 2% by moles of butene, the density is 0.922 when the melt index is 0.03, and it is 0.933 when the melt index is 50, while the density is 0.9265 when the melt index is 1.

The relatively low values of the ratio between polymerized alpha-olefin and density and, at the same time, the high melting point bring about particularly high mechanical properties in articles made of the copolymers, e.g., films.

In particular, the films exhibit a high impact strength and, contemporaneously, a high tensile strength.

The polymers are prepared by polymerizing in the gas phase, in a fluid bed or in a stirred reactor, mixtures of ethylene and of alpha-olefin CH 2 ═CHR, in which R has the meaning indicated hereinabove, in particular butene, with catalysts prepared from Ti compounds supported on Mg halides and from organometallic Al compounds, which are active in the polymerization of ethylene, but little or not at all active in the homopolymerization of the alpha-olefins.

The catalysts consist, in particular, of the reaction product of:

(A) an Al-alkyl compound, in particular an Al-trialkyl such as Al(C 2 H 5 ) 3 , Al(i-C 4 E 9 ) 3 or a dihalide of Al-alkyl, with

(B) the solid product of the reaction between:

(a) a tetravalnet Ti-alcoholate comprising at least a Ti-OR bond in which R is an alkyl, aryl or cycloalkyl radical with 1 to 18 C,

and a complex between:

(b) a compound X n Mg(OR) 2-n in which X is a halogen, R has the meaning specified in (a) 0≦n≦2, and

(c) an Al halide of formula X 2 AlR in which X and R have the meanings indicated above and R, furthermore, may be, also, a group CR' in which R' has the same meaning indicated hereinabove for R.

In particular, component (B) is the product of the reaction of an adduct between a Mg dihalide and a dihalide of Al-alkyl with a Ti compound of the type specified above.

The preferred Ti compounds are the Ti tetraalcoholates. Examples of representative compounds are Ti(O--n--C 4 H 9 ) 4 and Ti(O--n--C 3 H 7 ) 4 . Examples of other useful Ti compounds include: Ti(O--n--C 3 H 7 ) 2 Cl 2 , Ti(O--n--C 3 H 7 ) 2 Cl 2 , Ti(O--n--C 4 H 9 ) 2 Cl 2 , Ti(O--n--C 4 H 9 ) 3 Cl.

Mg dihalides, for example MgCl 2 , or a Mg dialcoholate are employed in particular as compound (b).

If a Mg dihalide is utilized, the compound is dissolved in a dihalide of Al-alkyl in amounts equal to 4-6 moles in respect to the Mg halide, operating at temperatures high enough to cause the dissolution (80°-100° C.).

The solution is then reacted with the Ti alcoholate; in this case a catalyst component (B) is obtained the particles of which have sizes ranging from about 10 to 30μ, and which results in a polymer in the form of particles having sizes ranging from about 100 to 500μ.

Catalysts of this type, which permit control of the polymer granulometry, are preferably utilized.

It is also possible to employ, with analogous results, the catalysts described in German Patent application No. P 2,822,809.

The preferred polymerization technique is the one of the fluid bed. The polymerization is carried out at pressures lower than 50 atm. and at temperatures lower than the softening point of the polymer and generally ranging from 50° to 110° C. Ethylene is fed in such ratio to the alpha-olefin as to obtain a polymer having the desired density. The polymerized alphaolefin corresponds to about 1/10th of the amount contained in the feeding mixture. The molecular weight regulation is effected according to known methods, for example with hydrogen.

The following examples are given to illustrate the present invention in more detail and are not intended as limiting.

The melting point is determined according to the method described in Canadian Patent No. 849,081.